Nature's Operations which Alan is competent to Study. 469 



ranges beyond a concentric sphere with less than a yard for 

 its radius. 



It is just possible that the inner portion of this extension 

 is necessary to represent man's present knowledge, that, in 

 fact, some of the non-gaseous nebulae, e.g. the great Nebula 

 in Andromeda, may be stellar "universes" distinct from 

 ours, and located somewhere within the larger sphere. Jf 

 so, when we looked upon the speck of light which brightened 

 up in the Nebula of Andromeda a few years ago, we may have 

 been then actual spectators of an event which really happened 

 some hundreds of thousands of years ago, the waves of wireless 

 telegraphy which communicated the information to us having 

 occupied the whole of that immense time upon their swift 

 journey. 



Of the Relation between Light and our Scale. 

 This leads us to consider the relation in which light stands 

 to our survey. It is useful to do so, since it gives unity to 



Yiz. 6. 



Stellar Distances. 



Planetary. 



000 000 000 000 000 



9 467 94 6 7 



^— is the distance "— is the distance which 



which light will travel in light will travel in one year, 

 one hundred millions of 

 years. 



our survey to consider how our table is related to light, which 

 in one direction reaches, by the minuteness of its waves, the 

 borderland of molecular magnitudes, and in the other direction, 

 by reason of its great speed, can traverse immense distances 

 in periods of time which we can grasp. The relationship is 

 exhibited in the lower section of fig. 1, which gives the times 

 which light must have to enable it to reach us from the dis- 

 tances represented by a unit in each of the indicated parts of 

 the table. The information there recorded may be supple- 

 mented by that added in fig. 6. 



On the Measurement of Time. 

 The same table may be employed for measuring time. 

 Intervals of time for the purposes of physical inquiry are 

 best measured by the distances over which light in the open 

 aether would travel in those periods. In this way measures 

 of distance become measures of duration upon that scale upon 

 which a metro-eight (which is the same as the centimo-ten) 

 represents one-third of a second — a scale which in practice is 

 found to be very convenient, especially for the study of mole- 

 cular physics. To represent a second of time on the diagram, 



